This invention relates to methods of, apparatuses for, and compositions of matter useful in, pretreating furnish to increase the resulting strength of paper sheet made from furnish containing large amounts of anionic trash. Various properties of paper products, including strength, opacity, smoothness, porosity, dimensional stability, pore size distribution, linting propensity, density, stiffness, formation and compressibility are primarily due to the bonds which exist between the cellulosic fibers in the paper. The bonding capability of these fibers is enhanced by the mechanical beating or refining step(s) of the papermaking process, during which the fibers are made more flexible and the available surface area is increased.
The strength of paper products is a property having three categories, referred to as dry strength, wet strength or rewetted strength, and wet web strength. Dry strength is the strength exhibited by the dry paper sheet, typically conditioned under constant humidity and room temperature prior to testing. Wet strength, or rewetted strength, is the strength exhibited by a paper sheet that has been fully dried and then rewetted with water prior to testing. Wet web strength is strength of a cellulosic fiber mat prior to drying to a paper product. Strength additives are compositions of matter effective at increasing one or more of these strengths.
Strength resins are polymers generally added at the wet end of the papermaking process to the cellulosic slurry, prior to the formation of the paper mat or sheet, to improve the strength characteristics of the paper product. Strength resins are generally believed to work by supplementing the number of inter-fiber bonds.
Dry strength additives are used to increase the dry strength of various paper products including paper, paperboard, tissues and others. Dry strength additives are particularly useful in the manufacture of paper products from recycled fibers, as recycling is known to have a weakening effect on the resulting paper. In addition, dry strength additives should reduce the amount of refining required to achieve a given dry strength for a given pulp, and the corresponding energy consumption required for refining and should not adversely affect the drainage rate of the cellulose web on the papermaking machine.
Various approaches for using polyacrylamides and other polymers to increase dry strength of paper products have been described in U.S. Pat. Nos. 6,315,866, 7,556,714, 2,884,057, and 5,338,406 and U.S. patent application Ser. No. 12/323,976. These methods however have been disappointing when furnish contains large amount of anionic trash such as old corrugated cardboard (OCC), mechanic pulps. It is believed that this is due to the exceptionally high number anionic moieties present in this furnish which prevent the strength aid from bonding with the paper fibers.
It is therefore useful and desirable to provide compositions, methods, and apparatuses useful in improving the effectiveness of strength aids in furnish containing large amounts of anionic trash. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “Prior Art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.